Obsessive-compulsive disorder (OCD) is a disabling and frequent neuropsychiatric disorder. Forty percent to 60% of patients with OCD fail to respond to available treatments. Neuroimaging studies have highlighted an association between the severity of obsessive and compulsive symptoms and an increased activity of the left orbitofrontal cortex (OFC) in patients with OCD. Transcranial direct current stimulation (tDCS) is a powerful and easy-to-use tool to modulate brain activity. Cathodal tDCS (c-tDCS) is assumed to decrease cortical excitability in the targeted brain region. We hypothesized that c-tDCS applied over the left OFC alleviates symptoms in patients with treatment-resistant OCD. We report here the case of a patient who received 10 sessions (2 mA, 20 minutes) of c-tDCS. The tDCS sessions were delivered twice a day with a 2-hour interval, with the cathode (35 cm²) placed over the left OFC and the anode (100 cm²) placed over the contralateral occipital region. No adverse event was reported. One month after the completion of the tDCS sessions, we observed a 26% reduction in severity of obsessive and compulsive symptoms measured using the Yale-Brown Obsessive Compulsive Scale scores. These findings are consistent with a previous study reporting a similar reduction in obsessive and compulsive symptoms after a low-frequency repetitive transcranial magnetic stimulation was given to the left OFC. Our results indicate that c-tDCS applied over the left OFC may be a suitable and safe treatment in patients with treatment-resistant OCD.